Containers made of foamed lightweight polystyrene sheet exhibiting excellent heat resistance, rigidity and buffering properties have been extensively used as containers for fast foods, such as hamburgers, fried chicken and carryout box lunches and as containers for instant foods, such as precooked Chinese noodles and soup mixes. These containers are prepared by heat-forming a foamed polystyrene sheet alone or a composite sheet comprising a laminate of a foamed polystyrene sheet and another resin film. However, since these containers have various defects, they are not satisfactory as food containers. More specifically, since foamed polystyrene sheet alone or a composite sheet composed of a polystyrene resin film and a foamed polystyrene sheet has poor heat resistance, food packaged in a container made of such materials cannot be directly heated by cooking in a microwave oven. And because of the inherent odor of polystyrene) its use is considerably limited Although a composite sheet comprising a laminate of a foamed polystyrene sheet with a polyethylene resin film or polypropylene resin film improves in the heat resistance to some extent over the above-mentioned laminate sheet, the heat resistance is still insufficient so that it is not suitable for containing of an oily foods to be cooked in a microwave oven, because the temperature of the food often exceeds 100.degree. C., a temperature which the above-mentioned composite sheet cannot withstand. Moreover, the above-mentioned olefins have a strong, so-called olefinic odor so that they might impair the taste or flavor of the food unfavorably. A composite sheet composed of a foamed polystyrene sheet and a nonoriented polyethylene terephthalate film is advantageous in that the taste and flavor of the food are not impaired, but its heat resistance is poor. A composite sheet composed of a foamed polystyrene sheet and a biaxially or monoaxially oriented polyethylene terephthalate film is excellent in the heat resistance and is tasteless and odorless. However, when such a composite sheet is heat-formed, the polyethylene terephthalate film layer causes delamination or breaks so that no containers cannot be satisfactorily prepared. Thus, the known composite sheets are insufficient for use as a heat-resistant food container.
According to the present invention, a polybutylene terephthalate resin film is providing having high heat resistance and rigidity, excellent formability, low oxygen permeability and excellent flavor retention. The polybutylene terephthalate film does not impair, the taste or aroma of the food in contact therewith because the film itself is tasteless and odorless. It has also been found that a composite sheet composed either of a foamed polystyrene sheet and a polybutylene terephthalate resin film, or of a foam polystyrene sheet, a polystyrene resin film and a polybutylene terephthalate resin film exhibits excellent heat resistance and heat-insulating properties, in addition to good formability properties which enable it to be formed into a container. And, since the oxygen permeability of such composite sheet is low, a container made of the same provides an excellent means for preserving foods.